1. Field of the Invention
Embodiments of the invention relate to the field of ion implantation of semiconductor structures. More particularly, the present invention relates to a plasma flood gun having a pinched outlet arrangement for generating and directing low energy plasma into contact with an ion beam.
2. Discussion of Related Art
Ion implantation is a process used to dope impurity ions into a substrate such as, for example, a semiconductor wafer. Generally, an ion beam is directed from an ion source chamber toward a substrate. Different feed gases are supplied to the ion source chamber to obtain plasma used to form ion beams having particular dopant characteristics. For example, from the feed gases PH3, BF3, or AsH3, various atomic and molecular ions are created within the ion source, and subsequently accelerated and mass selected. The depth of implantation of the generated ions into the substrate is based on the ion implant energy and the mass of the ions. One or more types of ion species may be implanted in the target wafer or substrate in different doses and at different energy levels to obtain desired device characteristics. A precise doping profile in the substrate is critical to proper device operation.
During the implantation process, bombardment of positively charged ions on the target substrate may result in the build-up of a positive charge on insulated portions of the wafer surface and lead to positive potentials thereon. The energetic ions can also contribute to further wafer charging through secondary electron emission from the wafer. The resulting positive potentials can create strong electric fields in some miniature structures, causing permanent damage. A plasma flood gun (PFG) can be used to alleviate this charge buildup. In particular, a PFG may typically be located near the platen close to the incoming ion beam just before it makes its impact on a wafer or target substrate. The PFG often comprises a plasma chamber wherein a plasma is generated through ionization of atoms of an inert gas such as argon (Ar), xenon (Xe) or krypton (Kr). Low-energy electrons from the plasma are introduced into the ion beam and drawn towards the target wafer to neutralize the excessively positively charged wafer.
Existing PFGs suffer from a number of drawbacks. One significant drawback is that of metal contamination. In particular, certain conventional PFGs use a hot tungsten filament for plasma generation. During operation, the tungsten filament is gradually consumed and tungsten atoms may contaminate the ion implantation system as well as the process wafers. Another common source of metal contaminants is the PFG plasma chamber itself. The inner surface of the plasma chamber often contains one or more metals or metal compounds. Constant exposure of the inner surface to plasma discharge may free metal atoms into the ion implantation system. Metal electrodes or other metal components placed inside the plasma chamber may cause similar contaminations.
Although the contamination problem might be alleviated by constructing a plasma chamber substantially out of a dielectric material, such a solution may not be desirable because the nonconductive inner surface increases plasma potential and consequently affects the energy of the emitted electrons. For charge neutralization in an ion implantation system, a relatively low electron energy is generally preferred. Low energy electrons can readily be trapped within the positive electric potential of the ion beam and then travel within the beam towards a positively charged wafer. In comparison, excessively energetic electrons can escape from the beam and do not arrive at the wafer. Also, excessively energetic electrons, if they arrive at the wafer, can lead to net negative charging on the wafer surface. This can result in the build-up of excess negative charge on the wafer surface where the degree to which such a negative electrostatic charge can accumulate on the wafer surface is related to the energy of the electrons arriving at the wafer.
A further challenge in designing a PFG is to make it compact enough to fit into a predefined space reserved for an existing PFG without requiring substantial modifications to existing ion implantation systems. It is often economically unfeasible to modify a mature ion implantation system just to accommodate a new PFG. Thus, upgrading a PFG for an otherwise operable ion implanter requires a PFG design that can easily be retrofitted into current systems. Thus, there is a need to provide a PFG which overcomes the above-described inadequacies and shortcomings.